(a) Field of the Invention
This invention generally relates to a system for transporting bulk materials to a ship, and more particularly, but not by way of limitation, to a shiploader system which incorporates a slewing boom to deliver bulk materials into the cargo hold areas of a ship.
(b) Discussion of Known Art
The transport of fungible materials such as ores or grains from one part of the world to another has been advantageously carried out by means of ships. Perhaps the greatest advantage of transporting these materials by ship has been the low cost associated with ocean transport. However one serious drawback to the use of ocean transport has been the cost and time delays associated with providing the infrastructure needed to load the vessels.
To evaluate the cost of a system one should focus on both the initial capital cost as well as the operating cost. The capital cost is generally determined by the capacity of the shiploading system, the size of vessel to be loaded and the type of shiploading system to be used. The type of shiploading system to be used determines the type of structure that must be installed to accommodate the ship and the loading system. Key components of the structure that must be installed includes the breasting structure, which includes breasting and mooring dolphins to accept the ship during loading. The system will also require support structure to accommodate the key components that make up the shiploading mechanism. Finally the system will also require conveyor structure for the approach conveyor system that feeds the shiploading mechanism.
Another concern associated with the selection of the type of shiploading system to be installed includes the lead time required for securing the shiploader system itself. This problem is particularly acute for large systems, since only a few manufacturers in the world have the facilities and technical ability to build these systems. The facilities and technology required to build these large systems results in lead times of as much as a year or more from the time of ordering the system to the time that the system is delivered for installation at the erection site.
Also of concern with shiploading systems are the costs of maintenance and the reliability of the systems. Thus, it is important to select a system that includes components that have been proven to be reliable mechanisms or components that may be purchased from a variety of manufacturers or suppliers. Therefore, the use large, complicated, custom components will undoubtedly result in increased risk of loss due to long downtime produced by long lead times needed for acquiring spare parts for specialty or custom fabricated systems.
Still another concern in the selection of a shiploading system is the system's ability to distribute the cargo to the different cargo holds of the vessel. Cargo distribution generally involves three important factors, these include distribution of cargo within each cargo hold, speed or rate of delivery to a cargo hold, and speed or ability to advance from one cargo hold to another cargo hold on the same vessel. The system's ability to distribute cargo within a cargo hold allows more efficient use of the space within the hold. Consequently, the vessel's carrying capacity can be improved by using a loading system that produces good cargo distribution. Perhaps the most immediately observable improvements from improved distribution is the increase in the efficiency of use of the cargo carrying space within the cargo bins or holds. (As used herein, the term cargo bin is synonymous with cargo hold.) Thus a system which can reach the furthest corners of the cargo bins will allow a more complete and uniform filling of the bin as compared with a system that can only reach a specific point or partial area of the cargo bin. It is important to fill the cargo bins by creating a generally flat, uniform pile of material. By stacking the cargo in a generally flat, uniform pattern, versus a generally cone shape produced by pouring the material from a single point, one maximizes the stability of the vessel. The added stability, which translates into added safety, is produced by the fact that a flat, uniform distribution of the cargo within the vessel will result in a center of gravity for the cargo that coincided with the center of gravity of the vessel. By producing a cargo load, or loaded shipment, where the center of gravity of the loaded shipment coincides with the center of gravity of the ship, one reduces the existence of an overturning moment produced by a distance between the center of gravity of loaded shipment and the center of gravity of the ship. Moreover, a flat, uniform distribution of the cargo will avoid shifting of the cargo during the voyage. By preventing shifting of the load one reduces the possibility of de-stabilizing the vessel during the voyage.
Another important aspect to consider in evaluating a shiploading system is the system's average loading rate instead of its nominal or design loading rate. The average loading rate for the system is the tons of cargo delivered to the ship divided by the amount of time that it took to fill the ship. Thus the average loading rate results in a statistic that reflects the overall efficiency of the system since it is a function of the size and speed of the conveyors used within the system, as well as a function of the steps that must be carried out in loading the vessel. Thus a system with conveyors of high capacity or high nominal rate may achieve a low average loading rate if these conveyors must be stopped frequently and for longer periods of time to allow the shifting of the loading system relative the vessel's position in order to provide access to the different bins of the vessel.
Since the time to load a vessel is unproductive use of the vessel, and may even present a constraint in the overall productivity of the system producing the fungible goods, is advantageous to minimize the time to load the vessel by increasing the average loading rate of the shiploading system for every vessel. The rate at which the vessel is loaded depends on both the material delivery rate (the nominal rate) and the system's ability to shift loading procedures from one cargo bin or hold to another. It is essential that the system loads the different cargo bins in a sequence that minimizes the possibility of damage to the vessel's structure and stability. For example, one typically begins loading at the ship's forward most cargo hold, since this hold is likely to be the highest point of the hull above the water due to the fact that this is the lightest section of the vessel.
The loading of the cargo holds will proceed by partially loading a cargo hold, and then proceeding to partially loading the next desired hold. This piecemeal, back and forth, loading is carried out in order to minimize the possibility of introducing a dangerous imbalance caused by a difference in the location of the center of gravity of the cargo and the center of gravity of the ship, as well as to prevent the possibility of damaging the vessel's structure, for example by placing large loads at the extremes of the vessel. Thus it is clear that an important characteristic of a shiploading system is its ability to load the vessel in a piecemeal fashion, while maintaining good overall loading rates.
Examples of known shiploader installation configurations include five basic types of shiploading systems. These systems include fixed loaders, traveling loaders, quadrant loaders, slewing/traversing loaders, and linear loaders.
Fixed loaders allow simple luffing or combined slewing and luffing type movement to distribute the loads to the different cargo holds of the vessel. The fixed loader is still used due to its simplicity and low acquisition and maintenance cost, but due to its limited movement and reach the fixed loader suffers from significant disadvantages. Perhaps the greatest disadvantage of the fixed loader is that it requires that the ship be shifted relative to the loader in order to allow filling of each of the different cargo holds. This exposes the ship and docking structure to the danger of accidental collisions. Moreover, the shifting of the ship's position is more time consuming than shifting a shiploader's position relative to the vessel, and thus the use of the fixed loader wastes valuable equipment time, resulting in a low average loading rate.
The traveling loader system is perhaps the oldest known alternative to a fixed loader and consists of a large, straight runway and a dock conveyor system that is mounted in a generally parallel fashion to the runway. To transfer cargo to a ship's cargo bin with this kind of system the cargo is first transported to the runway area by an approach conveyor. Then the approach conveyor transfers the load to the dock conveyor on the runway, and then from the dock conveyor the load is transferred to a boom that delivers the cargo to the ship's holding bin.
With the traveling shiploader system the ship is moored against dolphins that allow the ship to be held in a parallel orientation to the system's runway. Thus, to load the various holding bins of the ship, the boom system must be able to travel along the runway, with the boom in a generally perpendicular orientation to the runway. While the traveling shiploader systems have proven to be reliable and effective, they have limitations. Perhaps the greatest limitation of these systems is that they require extensive marine structure for the dock and runway of the shiploader system. This translates into large, costly installations with long construction lead times.
Another major limitation is that the feeding point of the conveyor system on a traveling loader is fixed along a line defined by the length of the shiploader's boom. Also, the shifting the boom from one cargo bin to the next can only be carried out in one direction without emptying the dock conveyor. For example, if the shiploader begins to deliver cargo to the bin closest to the stern of the ship (in situation where the ship is docked such that its stern is closest to the approach conveyor) it may shift to cargo bins that are successively closer to the bow of the ship. Due to the fact that the boom on the traveling loader may not reverse its direction of shifting without first stopping and unloading the entire contents of the dock conveyor before shifting the boom back towards the stern of the ship (towards the approach conveyor). This is due to the fact that as the boom is moved from one bin to the next, the transfer point from the dock conveyor to the boom must also shift to feed the boom at its new location. This shifting is typically carried out by incorporating what is known as a tripper system along the dock conveyor. The tripper system is a device that uses a set of idlers and pulleys to introduce an overhang or ripple into the belt. The cargo material on the dock conveyor separates from the conveyor as it passes over the overhang or ripple. The cargo material which separates from the dock conveyor is then received by the boom, which then transports the cargo material to the cargo bin.
Thus, the tripper mechanism must be shifted along the dock conveyor as the system shifts the loading to successive cargo bins. This movement of the tripper system is a relatively simple procedure as long as the boom moves in one direction along the runway. However, a problem arises when the direction of motion of the boom must be reversed. When the direction of motion of the boom is reversed the entire dock conveyor must be unloaded. The unloading of the dock conveyor requires that the delivery of material to the system be stopped; which results in a reduction in the average loading rate of the system.
The quadrant shiploader system, illustrated on FIGS. 1D and 1E, was an advancement over the traveling shiploader system in that it results in a substantial reduction in the marine structure requirements for its operation. The quadrant system uses a pivoting bridge of fixed span to distribute the cargo to the ship's cargo bins. With the quadrant shiploader, one end of the bridge is secured at a location where it can receive cargo from an approach conveyor. The mid section of the bridge is supported by a carriage system that rides over an arched runway. The second end of the bridge is cantilevered from the runway, with its boom cantilevered over the ships to be loaded, and thus serves for delivering cargo to the ship's cargo bin. An example of a shiploader that is similar to the quadrant type loader is taught in U.S. Pat. No. 4,082,181 to Berthold et al. Due to the similarities of the Berthold device to the quadrant type loaders, the Berthold device suffers from the same limitations as the quadrant loaders.
In order to load a ship with the quadrant loader, the ship must be moored in a manner that the center of the cargo bin area or the length of the ship is substantially normal to a line that extends from the center point of the cargo bin area or length of the ship to the pivot point of the bridge. It will become apparent that this arrangement is disadvantaged in that the sweep of the bridge causes the discharge of the boom to follow an arch, while loading a ship that has a straight, long hull. Thus, to load the ship's cargo bin, a quadrant loader must provide for adjustment of the combined length of the bridge and boom, so that the discharge of the boom follows a substantially straight path. The feed conveyor or system of the quadrant loader also has to be stopped every time the direction of shifting is to be reversed. (However the conveyor system does not need to be emptied.) Moreover, due to the fact that the runway support of the bridge and boom sweeps to and from the ship, the quadrant loader requires the longest reach, or cantilevering of the boom, for reaching the corners of the cargo bins, as compared to known loaders. However, while the quadrant loader has limitations concerning its reach of the bins, it generally produces higher average loading rates due to more efficient shifting from one bin to the next.
The slewing traversing shiploader is simply a traveling type shiploader with an additional pivot point, or slewing bearing, mounted on a carriage that rides on a runway that is parallel to the pier or docking area. This device combines the advantages and disadvantages of the quadrant loader and the traveling loader. Thus the mechanisms of the slewing traversing loader are somewhat more complicated than the mechanisms involved for each type of system alone.
An example of a slewing traversing type loader is taught in U.S. Pat. No. 3,499,522 to Novak. It follows from the discussion above that the Novak device, while incorporating the advantages of the quadrant and the traveling loader, suffers the disadvantage of cost, moderate average loading rates, and marine structural requirements of the traveling loader. Specifically, a significant disadvantage of the Novak device is that it requires the construction of two separate piers, one for an arched runway and one for a straight runway.
An approach at correcting the disadvantages of the quadrant shiploading system is taught in U.S. Pat. No. 3,856,159 to Soros. The Soros invention is now commonly known as the linear type loader since the bridge is supported over a straight runway which parallels the ship to be loaded. The linear loader is equipped with a variable span bridge. The land side of the bridge is allowed to shuttle on top of a slewing bearing, and the water side allowed to travel along a straight runway. This arrangement requires a shorter unsupported boom span to reach the corners of the cargo bins than is required by the quadrant type loaders. A linear type loader has been illustrated on FIGS. 1B and 1C.
One important disadvantage of the linear loader is that it requires a bridge adjustment mechanism in order to provide discharge of the cargo along a straight line. This adjustment mechanism means a substantial increase in the weight of its pivoting end. (Its overall weight, however, is somewhat less than the weight of a comparable quadrant loader.)
Yet another disadvantage of existing systems, such as the linear and quadrant systems, is that a traveling shuttle and boom conveyor is supported by a slewing bridge with large span and additional long cantilevers on both ends. The bridges are subjected to constant stress reversals induced by heavy dynamic loads of the boom and traveling shuttle. In case of the Linear loader this is further magnified by the fact that the bridge itself travels above the rear pivot. Therefore the bridges of both types of shiploaders have to be designed with very low depth to span ratio generating deep heavy sections with substantial weights.
There remains a need for a shiploading system that can be erected with short lead times. Thus, there remains a need for a shiploader system that can be assembled from non-specialty components and yet produce high average loading rates for small as well as large vessels.
There remains a need for a shiploader system that makes more efficient use of standard shiploading or materials handling equipment. More specifically, there remains a need for a shiploader system that reduces the size and amount of marine structure, while producing high average loading rates with readily available components.
Still further, there remains a need for a shiploader system that takes advantage of proven, readily available components to produce a versatile system that can accommodate large vessels as well as small vessels.
There remains a need for a shiploading system that requires low capital costs and takes advantage of systems with proven reliability to produce a shiploading system that can operate at higher average loading rates than had been previously known.